1. Field of the Invention
The present invention relates to a light modulation circuit for driving backlight of a liquid crystal display device.
2. Description of the Related Art
Generally, a cold cathode tube such as a fluorescent lamp is used as backlight of a liquid crystal display device. The backlight of the liquid crystal display device often needs to be modulated in view of battery life where the liquid crystal display device is incorporated in a small-sized portable apparatus, or in view of display enhancement where the display is used outdoor, indoor and in various other situations. A pulse width modulation (hereinafter, simply referred to as "PWM") technique is one of the known techniques for modulating the backlight. According to the PWM technique, a light source is periodically turned on and off while a ratio of ON and OFF periods of the light source is changed.
FIG. 7 is a circuit diagram showing a light modulation circuit disclosed in Japanese Laid-Open Publication No. 6-325890. Referring to FIG. 7, the light modulation circuit includes a light modulator 30, an oscillator 31, a transformer 13, a fluorescent lamp 14, a transistor Q3 and the like.
FIG. 8 is a waveform diagram showing waveforms of signals of the light modulation circuit shown in FIG. 7.
Referring to FIG. 8, when a mode of a signal S1 transits from a light non-modulation mode to a light modulation mode, the oscillator 31 generates a signal S2 which alternately repeats a high level and a low level. When the signal S2 is at the low level, the transistor Q3 is electrically connected, whereby the light modulation circuit shown in FIG. 7 generates signals S3 and S4.